With self-propelling work machines such as dump trucks, trucks or other self-propelling off-road vehicles for construction sites, mines and the like, electric drives having at least one electric motor have been used in recent times to utilize the typical advantages of such electric drives with respect to hydrostatic drives such as their better efficiency and an easier maintenance. Considerably lower operating costs can also be achieved at the partly substantial powers due to the substantially better efficiency. The electric drive can in this respect in particular be utilized as a traction drive by means of which at least one wheel or one chain drive of the undercarriage is driven, but also for driving a main work unit such as the milling drum of a surface miner.
In this respect, a generator can be provided for the power supply of the electric drive, said generator being drivable by an internal combustion engine, for example in the form of a diesel engine, of a gasoline engine or of a gas engine, wherein not only the power generator, but also a hydraulic unit, in particular its pump, can be driven by the internal combustion engine to be able to hydraulically drive other adjustment actuators of hydraulic components. With a bulldozer, the adjustment and/or lifting device for the trenching shovel can, for example, be driven by means of such hydraulic actuators. With dump trucks, the dump body can be rocked up and down by means of a hydraulic actuator.
A bulldozer having such a drive concept comprising an electric drive is known, for example, from U.S. Pat. No. 7,950,481, with it being proposed here to arrange an electric motor centrally and to transfer its drive power to different elements to be driven via a differential. It is proposed in this respect to store excess electrical energy which is generated by the generator with an internal combustion engine not utilized to capacity in a battery in order to be able to transfer additional electrical energy in the sense of a boost function to the electric motor when the latter requires a particularly high power, which may the case, for example, on the starting up of the machine. If, conversely, the work machine is to be braked, mechanical brakes in the form of spring pre-loaded disk brakes which can be hydraulically ventilated are actuated. Depending on the size of the work machine and on its purpose, such brakes have to be dimensioned more or less large in order not to overload or overheat on intensive braking procedures over a longer time such as can be the case with bulldozers constantly moving backward and forward or with fully loaded dump trucks traveling downhill.
U.S. Pat. No. 8,395,335 B2 furthermore describes an electric drive system for off-road trucks in which the electrical drive energy is provided by an internal combustion engine which drives a generator. In braking operation, electrical motor braking power provided by the electric motors is transferred to the generator to reduce the fuel consumption of the internal combustion engine. Excess electrical motor braking power is furthermore transferred past the internal combustion engine to electrical auxiliary units to drive these auxiliary units electrically and is finally dissipatively reduced or “burnt”, i.e. converted into heat, via electrical braking resistors in the form of a so-called grid box. The distribution of the electrical motor braking power, however, requires a relatively complicated control system while taking account of the electrical energy usable at the auxiliary units. In addition, the thermal load arising at the named grid box has to be taken into account.
It is the underlying object of the present invention to provide an improved work machine of the initially named kind as well as an improved method for braking such a work machine which avoid disadvantages of the prior art and further develop the latter in an advantageous manner. An energy-efficient braking with sufficient decelerations should preferably be made possible using a braking apparatus which is of a simple design and is easy to control.
The named object is achieved by a self-propelling work machine having an electric drive comprising at least one electric motor; the work machine further comprising a generator drivable by an internal combustion engine and for supplying the electric drive with electric current; the work machine further comprising a braking apparatus for braking the work machine, wherein the braking apparatus provides a regenerative braking by the electric drive and comprises a feedback apparatus for feedback of electrical motor braking power of the electric motor to the generator, wherein a braking control apparatus is provided for automatic connection of at least one mechanical brake in dependence on the electrical motor braking power fed back to the internal combustion engine and/or on an operating state of the internal combustion engine acted on by the fed back motor braking power, and by a method for braking such a work machine.
It is therefore proposed to achieve the desired braking effect primarily by regenerative braking via the electric motor or motors and to apply the electrical motor braking power generated in this process on the internal combustion engine and in so doing to take up or to prevent an overload of the internal combustion engine due to excessive fed back motor braking power by connecting a mechanical brake. In accordance with the invention, a braking control apparatus is provided for an automatic connection of a mechanical brake in dependence on the motor braking power fed back to the internal combustion engine and/or in dependence on the operating state of the internal combustion engine acted on by the fed back motor braking power. The braking energy or the electrical motor braking power provided by the at least one electric motor is advantageously primarily fed back to the internal combustion engine when coasting via the generator which converts the electrical motor braking power into mechanical drive power for the internal combustion engine and is used there for the drive of the secondary consumers such as fans, coolers or pumps connected to the internal combustion engine and for overcoming the drag resistances of the internal combustion engine. If the motor braking power applied on the internal combustion engine exceeds a degree compatible for the internal combustion engine and/or if the internal combustion engine reaches a predefined operating state under the effect of the fed back motor braking power, the mechanical brake is automatically connected to avoid or to reduce a further increase of the electrical motor braking power applied on the internal combustion engine. The connection of the mechanical brake can in this respect advantageously take place gently with—where required—successively increasing braking force so that the transition from a braking without a mechanical brake to a braking with a mechanical brake, and vice versa, takes place in the manner of a blending procedure in a gently transiting manner without a deceleration burst. The braking force of the mechanical brake can be gently varied and controlled, in particular regulated, while taking account of the braking power already applied on the internal combustion engine in order to come as close as possible to a desired braking force predefinable by the driver.
In this respect, braking advantageously only takes place using the mechanical brake when the motor braking power fed back to the internal combustion engine reaches the deceleration capacity of the internal combustion engine and of auxiliary units which may be connected thereto.
An increasing electrical motor braking power which is generated by the at least one electric motor can in particular first be applied on the internal combustion engine with an increasing desired or required braking power—for example by increasing actuation of a brake generator and/or by an increasing slope, with the fuel supply to the internal combustion engine being successively reduced until the internal combustion engine no longer consumes any fuel at a constant speed. As the fed back electrical motor braking power increases further, the internal combustion engine can advantageously be revved up beyond a constant engine speed desired per se until a maximum permitted or desired engine speed of the internal combustion engine is reached, with the named revving up of the internal combustion engine advantageously taking place with a blocked fuel supply. If the named maximum speed of the internal combustion engine is reached, the aforesaid braking control apparatus connects the mechanical brakes in order to take up fed back motor braking power increasing even further and hereby to prevent or at least to restrict a further increase in the fed back motor braking power transferred to the internal combustion engine.
The braking control apparatus therefore advantageously provides a plurality of braking stages which can be connected after one another to take up the desired or required braking power. Initially or primarily, electromotive motor braking power is applied on the internal combustion engine without connecting mechanical brakes in order to be able to operate the internal combustion engine in an energy-efficient manner, on the one hand, and to avoid an unnecessary thermal load or wear of the mechanical brakes, on the other hand. Only when a deceleration capacity of the internal combustion engine and of the auxiliary units connected thereto has been reached or when the compatibility limit of the application of the electrical motor braking power on the internal combustion engine has been reached are the mechanical brakes connected in a further stage. In this respect, within the aforesaid first braking stage in which the electrical motor braking power is only or at least primarily applied on the internal combustion engine, the fuel supply is in this respect initially reduced in a first sub-stage with a substantially constant internal combustion engine speed for so long until the fuel supply is completely cut off. Once the fuel supply is cut off, a revving of the internal combustion engine is permitted in a second sub-stage.
The braking control apparatus can advantageously control the braking power which can be applied on the internal combustion engine by varying the power pick-up of at least one auxiliary unit which is connected to the internal combustion engine, for example in the form of a fan, of a cooling apparatus or of a pump. The braking control apparatus can in particular increase the power pick-up of at least one such auxiliary unit before the mechanical brake is used. Not only the regenerative motor braking power and thus also the total braking power can hereby be increased an controlled in a more variable manner and can the wear of the mechanical brake be delayed, but above all an even more efficient operation of the work machine can also be achieved, for example in that a fan or a cooling apparatus is ramped up in a power respect beyond the degree required per se to cool corresponding units more than absolutely necessary so that then, with a subsequent ascent or also on a level path, the auxiliary unit can be switched off for longer or can be operated at a lower power than usual. In a further development of the invention, it is, however, likewise possible to operate an auxiliary unit such as a pump in a dissipative manner with a higher power pick-up in order to increase the regenerative motor braking power, for example by increasing the flow rate of the pump, for example by connecting a flow resistance.
The named braking control apparatus is configured in a further development of the invention such that the mechanical brakes remain unactuated or released for so long until the desired or required braking power can be applied on the internal combustion engine and optionally on auxiliary units connected thereto via electrical motor braking and feeding back the motor braking power, in particular for so long until the fed back motor braking power does not exceed a predefined limit value and/or the internal combustion engine acted on by the fed back motor braking power does not leave a predefined operating state or operating state range, in particular does not exceed a predefined engine speed.
In a further development of the invention, the feedback apparatus can manage without any electrical brake resistors, in particular free of so-called grid boxes. The feedback apparatus can hereby be configured and controlled simply overall and no thermal load limits at the electrical brake resistors have to be taken into account. The problems resulting with heavy precipitation, in particular of air-cooled brake resistors, can also be avoided, whereby the work machine becomes safer and can moreover also be of small construction. With air-cooled brake resistors, the danger is known of ground faults due to penetrating moisture, which can be avoided by a design of the feedback apparatus without braking resistors.
In order also not to allow a feedback power which is too high to arise on strong braking and to be able to reliably take up the portion of motor braking power which cannot be applied at the internal combustion engine, the connectable mechanical brake can be configured in an advantageous further development of the invention as a wet brake which can provide a sufficiently high braking power without thermally overheating. A mechanical multi-disk brake can in particular be provided which has oil cooling and which can be connected indirectly or directly to the drive element to be braked, for example to a wheel of the undercarriage or to a chain drive pinion. In a further development of the invention, a plurality of such mechanical brakes can be connected to a plurality of the drive elements; for example, corresponding wet brakes can be provided at each wheel of a truck.
In an advantageous further development of the invention, the electrical motor braking power generated in total by the electric motor or motors can be completely transferred to the generator connected to the internal combustion engine and the mechanical drive power generated by the generator in this process can be completely transferred to the internal combustion engine. The feedback apparatus can so-to-say be configured in one strand and cannot provide any branches or junctions which would transfer different portions of the fed back motor braking power to different drive elements. It would admittedly generally be possible to branch off a portion of the fed back motor braking power, for example directly in the form of electrical energy to electrical secondary units or not only to drive the internal combustion engine with the drive power of the generator, but also other drive elements coupled to the generator such as a secondary generator. With the aforesaid single-strand configuration of the feedback apparatus and with the substantially complete conversion of the fed back motor braking power into mechanical drive power for the internal combustion engine, however, no different systems have to be monitored, but the mechanical brake can rather only be connected substantially in dependence on the operating state of the internal combustion engine.
The brake control apparatus can in particular connect the mechanical brake in dependence on the engine speed of the internal combustion engine, and indeed in particular only when the speed of the internal combustion engine reaches a predefined maximum speed. The named brake control apparatus can for this purpose be connected to speed detection means which provide the named speed of the internal combustion engine.
The named brake control apparatus can furthermore comprise engine control means for reducing the fuel supply to the internal combustion engine which initially reduce the fuel supply at a constant speed of the internal combustion engine, in particular reduce it to an increasing degree such that with an increasing application of electrical motor braking power on the internal combustion engine the fuel supply is successively driven to zero and in so doing the engine speed is kept constant and/or at least at a predefined minimal speed, for example the idling speed.
The braking control apparatus can in this respect advantageously comprise braking force control means which resets or readjusts the braking force generated by the mechanical brake on reaching the maximum internal combustion engine speed, in particular such that, with a further increasing braking power requirement—for example due to a further increasing actuation of the braking generator—the braking force of the mechanical brake is further increased and with a further dropping braking power requirement the braking force of the mechanical brake is reduced, however, preferably only so much that the internal combustion engine is still held at its predefined maximum speed. The braking force of the mechanical brake is preferably regulated such that the maximum possible electrical motor braking power is applied on the internal combustion engine and on the optionally connected auxiliary units.
The present invention will be explained in more detail in the following with reference to a preferred embodiment and to associated FIG.